The present invention relates to a noise reducing circuit applied to a sound reproducing system having a structure in which a sound signal such as music or the like is reproduced in an electroacoustic transducer via a sound signal switching circuit and then a low-cut filter (a low frequency bandstop filter) for cutting off a low frequency range of the sound signal, and is intended to reduce a noise which occurs characteristically due to switching of the sound signal switching circuit in the above structure.
In a sound reproducing system for reproducing sound such as music or the like using a small-sized speaker, when a sound signal including a low frequency range is supplied to the speaker, the low frequency range of the sound signal, which does not contributes to reproduction, causes a vibration plate of the speaker to operate with a large amplitude so that there are a potential to narrow the movable range of the vibration plate with respect to a range not less than a mid-frequency range which contributes to the reproduction, and a potential for the excessive amplitude of the vibration plate to cause a damage to the vibration plate and a drive signal line. Accordingly, in the sound reproducing system, the frequency range amplified by an amplifier is limited so as not to supply a signal with a frequency lower than a frequency that can be handled by a low frequency range reproducing ability of the speaker to the speaker. FIG. 2 shows a related sound reproducing system in which the low frequency range is limited. A sound signal supplied from a sound source 10 such as a storing/recording medium reproducing device, a television/radio tuner, or the like passes through a sound signal switching unit 12. After that, a frequency component of the sound signal lower than, e.g., 1 kHz which does not contributes to reproduced sound is removed at a low-cut filter 14. Then, the sound signal is amplified in a power amplifier 16 and reproduced in a speaker (electroacoustic transducer) 18. The sound signal switching unit 12 is, e.g., a sound source selector, a sound signal cutoff switch (a mute switch or the like), or a volume for changing volume of the sound signal stepwise, and performs switching of the sound signal (switching among source types, on/off switching of the sound, stepwise changing of the volume, or the like) using a switching instructing unit 20 such as a switching operation element (a source switching operation element, a mute operation element, a volume changing operation element, or the like) switched by an operator, or a switching instruction signal outputting unit which automatically outputs a switching instruction signal without depending on the switching operation by the operator. The low-cut filter 14 includes a capacitor C1 and a resistor R1.
[Patent Document 1] JP-A-2007-259324
[Patent Document 2] JP-A-2001-203581
[Patent Document 3] JP-A-7-30497
[Patent Document 4] JP-B-4-78208
In the sound reproducing system having the structure of FIG. 2, a voltage Vc at both ends of the capacitor C1 of the low-cut filter 14 fluctuates in accordance with the frequency and the amplitude of the sound signal supplied from the sound source 10. Specifically, with regard to the sound signal having the frequency of not more than a cutoff frequency of the low-cut filter 14, as the frequency of the sound signal decreases, the voltage Vc increases. FIG. 3 show voltage waveforms in a case where the sound signal switching unit 12 is switched and the sound signal is cut off by the sound signal switching unit 12 when the sound signal having the frequency of not more than the cutoff frequency of the low-cut filter 14 is supplied from the sound source 10. FIG. 3 shows the sound signal supplied from the sound source 10 and a potential (the voltage at both ends of the resistor R1) at a connection point P of the capacitor C1 and the resistor R1. When the sound signal is cut off by the sound signal switching unit 12, the voltage Vc at both ends of the capacitor C1 in applied to the resistor R1 so that the potential at the connection point P of the capacitor C1 and the resistor R1 is reduced to—Vc. Thereafter, a charge of the capacitor C1 is discharged via the resistor R1 so that the potential at the connection point P of the capacitor C1 and the resistor R1 returns to 0 V with a time constant of R1 and C1.
Thus, in the sound reproducing system of FIG. 2, when the sound signal switching unit 12 is switched, the charge charged in the low-cut filter 14 is discharged, and an undershoot thereby occurs in the potential at the connection point P of the capacitor C1 and the resistor R1. The frequency component of the undershoot portion is away from the sound signal supplied from the sound source 10 toward a higher-frequency range so that a noise sound (a petit noise) which is not included in the sound signal originally, makes a petit sound, and is easily audible to ears is caused to emit from the speaker 18.
The case where the sound signal switching unit 12 cuts off the sound signal is described in the above. In a case where the sound signal switching unit 12 changes the volume of the sound signal stepwise (step by step), the similar phenomenon occurs. Specifically, the noise sound which makes the petit sound continuously occurs while the volume is being changed. For example, in a cellular phone in which the cutoff frequency of the low-cut filter 14 is set to 1 kHz such that the sound signal of not more than 1 kHz can not be reproduced, when the sound signal of 100 Hz is supplied from the sound source 10, the sound signal is removed in the low-cut filter 14, and is not reproduced through the speaker 18. However, at this time, when the sound signal switching unit (volume adjustment) 12 for changing the volume stepwise is operated to adjust the volume, the stepwise changing of the volume causes stepwise changes in the potential at the connection point P of the capacitor C1 and the resistor R1, and the undershoot is caused to occur at the connection point P when each change is made. Because of this, while the volume is being adjusted, the petit noise of not less than 1 kHz is audible. As the related art for preventing the occurrence of the noise sound resulting from the switching of the sound signal, there have been technologies described in Patent Documents 1 to 4.